Behind Tolerance and the Cost of Coexistence: The Cd–Mn Interaction in Sunflower Triggers Translocation Reconfiguration, Impacting Productivity and Food Security

Soil bioavailability influences plant uptake of chemical elements, but accumulation and translocation vary among species, particularly in tolerant plants such as sunflower. Because essential and potentially toxic trace elements persist in soils and enter the food chain, this study investigated the Cd-Mn interaction beyond apparent tolerance, evaluating its effects on metal translocation, photosynthetic efficiency, productivity, and food safety. Chlorophyll fluorescence, gas exchange, conversion efficiency, dry matter, oil content, pigments, and Cd and Mn concentrations in soil, roots, leaves, and seeds were evaluated. Mn uptake decreased in the presence of Cd, whereas Cd uptake increased with Mn. Mn accumulated mainly in leaves, while Cd remained in roots but showed greater translocation to shoots with Mn. Despite preserved photosynthetic function, high Mn reduced seed dry matter, conversion efficiency, and oil content, with smaller reductions under combined Cd exposure. Cd translocation to seeds raises food safety and biomagnification concerns.